This invention relates to methods for producing probes for use in probe-based instruments, including applications where high-speed imaging (up to video rate) is desired.
For the sake of convenience, the current description focuses on probes that may be realized for a particular embodiment of probe-based instruments, the atomic force microscope (AFM). Probe-based instruments include such instruments as AFMs, 3D molecular force probe instruments, high-resolution profilometers (including mechanical stylus profilometers), surface modification instruments, chemical or biological sensing probes, and micro-actuated devices. The probes described herein may be realized for such other probe-based instruments.
An AFM is an instrument used to produce images of surface topography (and/or other sample characteristics) based on information obtained from scanning (e.g., rastering) a sharp tip on the end of a cantilever relative to the surface of the sample. Topographical and/or other features of the surface are detected by sensing changes in the probe's mechanical response to surface features and using feedback to return the system to a reference state. By scanning the probe relative to the sample, a “map” of the sample topography or other sample characteristics may be obtained.
Changes in the probe's mechanical response are typically detected by an optical lever arrangement whereby a light beam is directed onto the cantilever in the same reference frame as the optical lever. The beam reflected from the cantilever illuminates a position sensitive detector (PSD). As the probe's mechanical response changes, a change in the output from the PSD is induced. These changes in the PSD signal are typically used to trigger a change in the vertical position of the base of the probe relative to the sample (referred to herein as a change in the Z position, where Z is generally orthogonal to the XY plane defined by the sample), in order to maintain a constant pre-set value for one or more of the probe's mechanical responses. It is this feedback that is typically used to generate an AFM image.
AFMs can be operated in a number of different sample characterization modes, including contact mode where the tip of the probe is in constant contact with the sample surface, and AC modes where the tip makes no contact or only intermittent contact with the surface. These two modes define two mechanical responses of the probe that can be used in the feedback loop which allow the user to set a probe-based operational parameter for system feedback.
In contact mode the interaction between the probe and the sample surface induces a discernable effect on a probe-based operational parameter, such as the cantilever deflection. In AC mode the effects of interest include the cantilever oscillation amplitude, the phase of the cantilever oscillation relative to the signal driving the oscillation, or the frequency of the cantilever oscillation. All of these probe-based operational parameters are detectable by a PSD and the resultant PSD signal is used as a feedback control signal for the Z actuator to maintain the designated probe-based operational parameter constant.
The feedback control signal also provides a measurement of the sample characteristic of interest. For example, when the designated parameter in an AC mode is oscillation amplitude, the feedback signal may be used to maintain the amplitude of cantilever oscillation constant to measure changes in the height of the sample surface or other sample characteristics.
Some current AFMs can take images up to 100 um2, but are typically used in the 1-10 um2 regime. Such images typically require 4-10 minutes to acquire. Many efforts are currently being made to move toward video rate imaging. The reasons for these efforts include the desire to image moving samples, to image more ephemeral events and simply to complete imaging on a more timely basis. One important means for moving toward video rate imaging is to decrease the mass of the probe, thereby achieving a lower spring constant with a higher resonant frequency.
Currently, conventional probes are 50-450μ in length with spring constants of 0.01-200 N/m and fundamental resonant frequencies, fR, of 10-500 kHz. Physical laws put lower limits on the achievable resolution and scan speed of conventional probes, given acceptable noise levels.
To get the best resolution measurements, one wants the tip of the probe to exert only a low force on the sample. In biology, for example, one often deals with samples that are so soft that forces above 10 pN can modify or damage the sample. This also holds true for high resolution measurements on ‘hard’ samples such as inorganic crystals, since higher forces have the effect of pushing the tip into the sample, increasing the interaction area and thus lowering the resolution. For a given deflection of the probe, the force increases with the spring constant, k, of the probe. When operating in air in AC modes where the tip makes only intermittent contact with the sample surface, spring constants below 30 N/m are desirable. For general operation in fluid, very small spring constants (less then about 0.1 N/m) are desirable.
To get measurements with higher scan speeds, one wants probes with a high fR. After passing over a sample feature, the probe response is about 1/fR seconds for contact mode and Q/fR seconds for AC modes (where Q is the quality factor for the probe). This sets a fundamental limit on scanning speed: if the response time of the probe is to be lowered, the fR must be raised.
The thermal noise of a probe involves fixed noise energy (of order kT) spread over a frequency range up to approximately the fR, where k is the Boltzmann constant and T is the temperature in Kelvin. Thus, the higher fR, the lower the noise per unit band width below fR.
The ideal probe for video rate imaging would have a fR in the 5-10 MHz range. It would also have a force constant in the 1-40 N/m range. Conventional probes would need to shrink an order of magnitude, to approximately 5-8 um in length or width, to achieve this goal.
Probes are microfabricated by using semiconductor integrated circuit fabrication techniques as this provides a way to batch produce probes with consistent cantilever and tip geometries necessary for use with AFMs today. These techniques include, but are not limited to: thin film deposition, photolithography with optical masks, Reactive Ion Etching (RIE) with plasma, wet etching of silicon, and wafer-to-wafer bonding. Silicon and silicon nitride are the two primary semiconductor materials from which AFM probes are fabricated. Silicon probes have thicker cantilevers which give higher resonant frequencies and force constants than silicon nitride probes. This is due to larger thickness variations when etching bulk silicon compared to depositing silicon nitride with Chemical Vapor Deposition (CVD), forcing silicon processes to stop at thicker cantilevers in order to assure higher yields. One can overcome these difficulties by using a Silicon-on-Insulator (SOI) wafer, but this introduces much higher costs. Silicon nitride probes have duller and shorter tips than silicon probes because silicon nitride is deposited in silicon molds which are difficult to machine and work with.
Current probe fabrication processes limit the ability of the person skilled in the art to reproducibly shrink probe lengths to 5-8 um, as well as their ability to shrink probe widths to those dimensions when probe lengths are also relatively small. This is due to a number of factors, including: (i) photolithography alignment issues when processing both sides of a silicon wafer, (ii) wafer bonding alignment issues, or (iii) photolithography variations on drastically uneven wafer surfaces. Probe fabrication processes usually incorporate at least one of these techniques and dimension variations of 5 um are not unusual. Furthermore, shorter probes will require the relatively thin cantilevers in order to keep force constants in the range required for AFM. All these factors make current processes unviable for the purpose envisioned here.